A 3rd Generation Partnership Project (3GPP) wireless communication system based on Wideband Code Division Multiple Access (WCDMA) radio access technology is widely deployed worldwide. High Speed Downlink Packet Access (HSDPA) that may be defined as the first evolution stage of WCDMA provides 3GPP with radio access technology which has high competiveness in the mid-term future.
There is an E-UMTS for providing high competiveness in the long-term future. The E-UMTS is a system evolved from the existing WCDMA UMTS and is being standardized in 3GPP. The E-UMTS is also called a Long Term Evolution (LTE) system. For detailed contents of the technical specifications of the UMTS and E-UMTS, reference can be made to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.
The E-UMTS basically includes User Equipment (UE), a base station, and an Access Gateway (AG) placed at the end of a network (E-UTRAN) and connected to an external network. In general, a base station can transmit multiple data streams at the same time for broadcast service, multicast service and/or unicast service. In an LTE system, Orthogonal Frequency Divisional Multiplexing (OFDM) and Multi-Input Multi-Out (MIMO) are used in order to transmit a variety of services in downlink.
OFDM represents a high-speed data downlink access system. An advantage of OFDM is high spectrum efficiency that enables all allocated spectra to be used by all base stations. In OFDM modulation, a transmission band is classified into a plurality of orthogonal subcarriers in the frequency domain and classified into a plurality of symbols in the time domain. In OFDM, a bandwidth per subcarrier is reduced and a modulation time per carrier is increased because a transmission band is split into a plurality of subcarriers. Since the plurality of subcarriers is transmitted in parallel, the digital data or symbol transfer rate of a specific subcarrier is lower than those of a single carrier.
A Multi-Input Multi-Out (MIMO) system is a communication system that uses a plurality of transmission and reception antennas.
A MIMO system can increase a channel capacity linearly without increasing an additional frequency bandwidth according to an increase in the number of transmission and reception antennas. MIMO technology includes a spatial diversity method capable of increasing transmission reliability using a symbol which has passed through a variety of channel paths and a spatial multiplexing method of increasing the transfer rate in such a manner that antennas transmit different data streams at the same time using a plurality of transmission antennas.
MIMO technology may be chiefly divided into open-loop MIMO technology and closed-loop MIMO technology depending on which a transmission stage knows channel information or not. In the open-loop MIMO technology, a transmission stage does not know channel information. Examples of the open-loop MIMO technology include Per Antenna Rate Control (PARC), Per Common Basis Rate Control (PCBRC), BLAST, STTC, and random beamforming. In contrast, in the closed-loop MIMO technology, a transmission stage knows channel information. The performance of a closed-loop MIMO system depends on how exactly is the channel information known. Examples of the closed-loop MIMO technology include Per Stream Rate Control (PSRC) and TxAA.
Channel information means information (e.g., attenuation, phase shift or time delay) about a wireless channel between a plurality of transmission antennas and a plurality of reception antennas. In a MIMO system, there are a variety of stream paths according to a plurality of transmission and reception antenna combinations, and a channel state has a fading characteristic in which the channel state is irregularly changed in the time/frequency domains over time due to time delay. Accordingly, a transmission stage calculates channel information through channel estimation. Channel estimation is to estimate channel information necessary to restore a distorted transmission signal. For example, channel estimation refers to the estimation of the size of a carrier and a reference phase. That is, channel estimation is to estimate the frequency response of a radio section or a radio channel.
A channel estimation method includes a method of estimating a reference value based on the Reference Signals (RSs) of several base stations using a two-dimensional channel estimator. Here, an RS refers to a symbol which does not have data actually, but has high output in order to help in obtaining carrier phase synchronization and base station information. The transmission side and the reception side can perform channel estimation using the RS. In channel estimation using an RS, a channel is estimated through a symbol that is known to both transmission and reception sides, and data is restored using the estimated value. An RS is also called a pilot.
FIG. 1 shows the structure of a receiver which supports MIMO.
As can be seen with reference to FIG. 1, the receiver which supports MIMO includes a plurality of antennas, a plurality of Low Noise Amplifiers (LNAs), a plurality of mixers, a plurality of Analog Digital Converters (ADCs), an oscillator (OSC), and a Phase-Locked Loop (PLL).
Signals Tx1, Tx2, . . . , Txn received from the respective antennas are amplified by the respective LNAs. The phase of a signal from the OSC is locked by the PLL, and the signal is distributed to the mixers. The mixers compose the signals from the LNAs and output the composed signals to the respective ADCs. The ADCs convert the respective signals into digital signals and output the digital signals as baseband signals, that is, B1, B2, . . . , Bn.
Meanwhile, in mobile UE including the MIMO receiver as described above, channel measurement is performed in the reception units at the same time in order to use an optimal wireless channel, and representative values of values measured by the respective reception units are transmitted to a base station.
Meanwhile, in channel measurement, a variety of items, such as a Received Signal Strength Indicator (RSSI), may be measured in order to configure an optimal communication environment and utilize radio resources efficiently.
In the case of the MIMO UE, the reception units perform measurement independently using the measurement items, calculate representative values, and report them to a higher layer. An equation therefore is as follows.m=f(m1,m2, . . . ,mn)  Equation 1
In the above equation, m1, m2, and mn are instantaneous measurement values measured by the first, second, and nth reception units of a plurality of reception units, f( ) means a statistical function, such as a maximum value or the mean value according to each measurement item, m is a representative value of a corresponding measurement item calculated through f( ). A higher layer performs upper filtering on the representative value additionally in order to reduce an error probability and uses the resulting value to perform optimization for the use of radio resources, such as handover or resource allocation.
A measurement item used in this radio transmission technology is basically divided into intra-frequency measurement and inter-frequency measurement.
Intra-frequency measurement is measurement for a frequency that is now being used. Since a reception unit including a baseband and a Radio Frequency (RF) is already set to a frequency now being used, the intra-frequency measurement can be performed without any influence while service is used.
In contrast, the inter-frequency measurement is measurement for a frequency different from a frequency that is now being used. In this measurement, an interruption of service that is being used, including a call, is indispensable during a measurement section due to limited embodiments if there is no additional reception unit.
Accordingly, a form in which a measurement gap is used, as in compressed mode in UMTS WCDMA, is being discussed and consideration to the form is included in a standard, but there are problems, such as a service stop or deteriorated call quality.
Meanwhile, the problems of inter-frequency measurement are recently actively discussed in standard LTE-A in order to solve the problems.
In the case of the LTE-A, in order to realize high-speed radio transmission, a variety of schemes including uplink MIMO and a Carrier Aggregation (CA) have been added. The CA is classified into an intra-band contiguous CA, an intra-band non-contiguous CA, and an inter-band non-contiguous CA, and a UE architecture that may be embodied is limited depending on each CA function. Basically, a UE architecture which supports a CA requires transceiver units equal to the number of Component Carriers (CCs) of a CA that may be supported at the same time. However, in the case of the intra-band contiguous CA, the application of a single-RF structure using the transceiver unit of a wideband which can support all CA bands at the same time is being actively discussed by taking several advantages in embodying UE into consideration.
Furthermore, in the case of a CA, the remaining Secondary Cells (S-Cells) other than a Primary Cell (P-Cell) are being standardized in such a manner that they are configured to be frequently activated/deactivated depending on the amount of necessary channels and user data in order to reduce the power consumption of UE. Thus, there is a need for frequent measurement for deactivated S-Cells. The measurement for deactivated S-Cells may be considered as a kind of inter-frequency measurement. In this CA, in the case of an intra-band non-contiguous CA and an inter-band CA which basically uses hardware in parallel, measurement for deactivated S-Cells can be performed by activating a deactivated RF chain without an additional service stop.
In contrast, in the case of intra-band contiguous CA technology in which a single RF chain is expected to be chiefly used, inter-frequency measurement for deactivated S-Cells requires the retuning of each reception unit including the retuning of a baseband and an RF, and thus a service stop occurs during this section.
FIG. 2 shows a state when UE capable of supporting MIMO and receiving only one frequency at once performs measurement for each reception unit.
As can be seen with reference to FIG. 2, the UE may use service using a first frequency f1 and the UE has to perform measurement for a second frequency f2. A section C1 is a section in which service is now being performed, and T12 and T21 mean respective sections in which the retuning of a reception unit is performed between the first and the second frequencies f1 and f2.
Furthermore, a section M2 indicates that the UE is performing actual channel quality measurement for the second frequency f2. The frequency of a carrier used in a frequency-down converter for each reception unit is indicated on the upper side of a section that corresponds to each reception unit.
The UE can support MIMO and receive only one frequency at once, and thus service through the first frequency f1 is stopped in the sections T12, M2, and T21. An ideal transfer rate of the UE is shown at the bottom of FIG. 2.
This type of measurement can be seen from common inter-frequency measurement, such as compressed mode of WCDMA.
FIG. 3 shows a state when UE capable of supporting MIMO and receiving two or more frequencies at once at the same time performs measurement for each reception unit.
As can be seen with reference to FIG. 3, the UE can perform service using a first frequency f1 and has to perform measurement for a second frequency f2.
Here, a section C1 is a section in which service is now being performed, and sections T12 and T21 indicate respective sections in which the retuning of a reception unit is performed between the first frequency f1 and the second frequency f2. C1M2 is a section in which service may be used using the first frequency f1, but measurement for the second frequency f2 is performed.
The frequency of a carrier used in a frequency-down converter for each reception unit is indicated on the upper side of a section that corresponds to each reception unit.
The UE can support MIMO and receive only one frequency, and thus an actual call stop is generated only in the sections T12 and T21 in which a reception unit including the retuning of a baseband and an RF is generated.
An ideal transfer rate of the UE is presented at the bottom of FIG. 3.
As can be seen with reference to FIG. 3, the UE capable of receiving one frequency at once has a smaller call interruption section than that of FIG. 2, but still includes a call interruption section.